Single-photon-emission apparatus

ABSTRACT

The apparatus includes a photon pair source for generating a photon pair that contains a signal photon and an idler photon and correlates with the generating time, photon detectors for detecting a incidence of idler photons, a clock generator, a gate device controller for generating signals for opening or closing a gate device in a frequency lowering a specific number of times within a specified time defined by the clock, and a gate device for opening or closing the gate in response to the signals from the gate device controller.

TECHNICAL FIELD

The present invention relates to a photon source used for as an example,quantum cryptographic communication system, a transmission system thatenables the detection of an unauthorized listener by loading each photonwith information.

BACKGROUND ART

In the quantum cryptographic communication system, loading each photonwith information enables the detection of an unauthorized listener bythe quantum mechanical principle. However, if the same information isloaded on two or more photons, the unauthorized listener may utilize apart of these photons and the presence of the unauthorized listener maynot be able to be detected. In this way, ideally, a pulse that containsonly one photon at maximum must be used. For this kind of pulse, it ispopularly practiced to attenuate the light beam from the laser beamsource by an attenuator in such a manner that the mean number μofphotons per pulse becomes about 0.1. By doing this, the probability tocontain two or more photons in a pulse can be reduced. However, theprobability to contain one photon in the pulse is also reduced to about0.1. That is, in the case of μ=0.1, transmission is actually carried outonly about once per 10 times.

Referring now to FIG. 9, description will be made on the case stated inthe “Key Distribution system and method using Quantum Cryptography” ofJapanese Unexamined Patent Publication No. 505019/1996 as one example ofconventional techniques for improving this kind of process. In FIG. 9,numeral 9 denotes a laser that generates pumping light for pumping thenonlinear optical crystal 11. In the nonlinear optical crystal 11, aparametric fluorescence pair that causes one photon of the pumping lightto stochastically generate two photons is generated. One photon of these(in this case, called the “idler photon”) is detected by an opticaldetector and a gate controller 49, and when detected, the gate device 4is opened to enable the other photon (called the “signal photon”) topass.

However, in the conventional technique, there are following problems.

First of all, the conventional method has a drawback in that if twophoton pairs exist within the response time of the detector, two signalphotons are emitted by a gate operation and two photons exist in apulse.

In the conventional method, it was unable to control the timing ofphoton generation within the pulse.

When the detector for detecting the arrival of the photon generates aso-called “dark count pulse,” that is, when the detector outputs pulsesdue to noises, etc. even when it does not detect the photon, it outputsnon-existent light pulse in which no emission photon exists and providedpoor efficiency.

The present invention has been made to solve these problems, and it isan object of the present invention to generate only one photon in onepulse.

It is another object of the present invention to reduce generation ofnon-existent beam pulse due to dark count pulse of the detector.

It is yet another object of the present invention to generate the photonat a specific timing.

DISCLOSURE OF INVENTION

A single photon generating apparatus according to the present inventioncomprises a photon pair source for generating a pair of photonsconsisting of a signal photon and an idler photon that correlate with agenerating time, a photon detector for detecting an incidence of theidler photon, a clock generator, a gate device controller for generatingthe signal for operating a gate device for the number of times less thana specific number of times within a predetermined time defined by theclock, and a gate device that opens and closes response to the signalfrom the gate device controller.

It also comprises a photon pair source for generating a pair of photonsconsisting of a signal photon and an idler photon that correlate with agenerating time, a photon detector for detecting the incidence of theidler photon, a clock generator, a gate device controller for generatingthe signal for operating the gate device only against a first signalfrom the photon detector within a predetermined time defined by theclock, and a gate device that opens and closes in response to the signalfrom the gate device controller.

For the nonlinear optical medium on which a pumping light is incident, anonlinear optical crystal is provided, in which the angle made by thepumping light and an optical axis of the nonlinear optical medium is setto the angle at which tuning curves come in contact with a straight linethat corresponds to a specific single wavelength a.

For the nonlinear optical medium on which a pumping light is incident, anonlinear optical crystal is provided, in which the angle made by thepumping light and the optical axis of the nonlinear optical medium isset to an angle at which tuning curves come in contact with two straightlines that correspond to two specific wavelengths a and b.

In addition, for a nonlinear optical medium on which a pumping light isincident, a wave guiding channel type nonlinear optical medium isprovided.

Furthermore, for a nonlinear optical medium on which a pumping light isincident, pseudo-phase matching type nonlinear optical medium isprovided.

In addition, as a gate device for controlling an emission of the signalphoton, a plurality of shutters for closing or opening in a timedifference shorter than a gate opening or closing time are equipped.

In addition, an optical fiber for allowing the signal photon caused ofthe photon pair to reach the gate device for controlling the emission ofthe photon is equipped.

In the present invention, an incidence of the idler photon is detectedby a photon detector and the gate device is opened or closed forcontrolling the emission of the signal photon for the number of timesless than the specific number of times within the predetermined timedefined by the clock from the clock generator.

In addition, the incidence of the idler photon is detected by a photondetector, and a gate device is opened or closed for controlling theemission of signal photon only for the first signal from the photondetector within the predetermined time defined by the clock from theclock generator.

In addition, the pumping light from the pumping light source is allowedto be incident and a photon pair which are generated by the nonlinearoptical medium that correlate with the generation time is used as anidler photon and a signal photon.

In addition, in installing the nonlinear optical medium on which thepumping light is incident, the angle made by the pumping light and theoptical axis of the nonlinear optical medium is set to an angle in whichtuning curves come in contact with a straight line that corresponds to aspecific single wavelength a.

In addition, in installing the nonlinear optical medium on which thepumping light is incident, the angle made by the pumping light and theoptical axis of the nonlinear optical medium is set to an angle in whichtuning curves come in contact with two straight lines that correspond totwo specific wavelengths a and b.

In addition, the pumping light is allowed to be incident to the waveguiding channel type nonlinear optical medium.

In addition, the pumping light is allowed to be incident to the pseudophase matching type nonlinear optical medium.

The emission of signal photons is controlled by a plurality of shuttersthat open or close in a shorter time difference than the open or closetime of the shutters.

In addition, the signal photon caused of the photon pair is allowed toreach the gate device that controls the emission of the photon by theuse of optical fiber.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a general arrangement drawing of one embodiment according tothe present invention;

FIG. 2 is a general arrangement drawing of one embodiment of the presentinvention;

FIG. 3 is a drawing showing the relationship between the wave length andemission angle of photons generated in the nonlinear optical medium;

FIG. 4 is a schematic drawing for explaining the operation of oneembodiment of the invention;

FIG. 5 is a schematic drawing for explaining the operation of oneembodiment of the invention;

FIG. 6 is a general arrangement drawing of one embodiment of theinvention;

FIG. 7 is an arrangement drawing of a gate device used in one embodimentof the invention;

FIG. 8 is a schematic drawing for explaining operation of the gatedevice used in one embodiment of the invention; and

FIG. 9 is a general arrangement drawing of one example of conventionaltechniques.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1

FIG. 1 is a general arrangement drawing of one embodiment of theinvention. In FIG. 1, numeral 1 denotes a photon pair source thatgenerates a photon pair that correlate with the generation time, numeral2 denotes a photon detector for detecting the idler photon 5, numeral 3denotes a differential circuit for differentiating the signal pulsegenerated from the photon detector, numeral 8 a gate device controllersection for controlling the gate device 4 in response to the signal fromthe differentiation circuit 3 and the control clock from the clockgenerator 7.

FIG. 2 shows a detailed configuration of the present embodiment. In thisembodiment, an optical pulse that contains only single photon isefficiently generated at a specific timing in the clock.

(Description on Photon Pair Generator)

In FIG. 2, numeral 9 is a light source of pumping light 10 for pumpingthe nonlinear optical medium 11. In the nonlinear optical medium 9, theidler photon 5 and the signal photon 6 that have wavelength 2λ, doublethe wavelength λ of pumping light 10, are generated by down conversion.In this embodiment, argon laser that has 351.1 nm wavelength is used forthe pumping light source 9. In this event, the idler photon 5 and thesignal photon 6 are generated as a pair, the sum of whose energiesgenerated is equivalent to the energy of the photon of 351.1 nmwavelength, that is, a photon of 702.2 nm wavelength, respectively.

As described in detail in the Japanese Patent Application No.353078/1997 “Photon Beam Generator,” setting the optical axis of thenonlinear optical medium to a specific angle with respect to the pumpinglight enables the idler photon 5 and signal photon 6 to be generated ina form of beam as well as in a high efficiency. FIG. 3 shows a tuningcurve when the optical axis of β-Barium-Boron-Oxide (BBO) crystal is setto the angle of 50.4° with respect to the pumping light. In FIG. 3, thewavelength of the photon generated is taken as abscissa and the emissiondirection of the photon with respect to the incident direction of thepumping light is taken as ordinate. As seen in the figure, two tuningcurves come in contact with a straight line that corresponds to the702.2 nm wavelength. Under this condition, a 702.2-nm wavelengthfluorescent pairs are emitted in a form of beam in directions of plus 3°and minus 3°, respectively. By using this kind of nonlinear opticalmedium, photon pairs are generated efficiently with respect to theincident power of the pumping light, and as a result, when the singlephoton is generated at an equivalent rate, it is possible to suppressthe power consumption of the equipment to a low level.

The idler photon 5 is focused by a lens 15, and converged into thephoton number detector 2 through the filter 17 that selectivelypenetrates the photon of 2λ wavelenght.

(Description on Photon Detector)

In this embodiment, as a photon detector 2, SPCM-AQ commerciallyavailable from SEIKO EG&G was used. This photon detector has anavalanche photo-diode (APD) driven in the Geiger mode of activequenching as a photo-receiving element. APD causes the breakdown statein which when a voltage exceeding a specified voltage (breakdownvoltage) is applied, incidence of only a single photon causes theinternal carrier induced by the incidence to be accelerated by theapplied voltage and repeats the process for exciting other carriersendlessly. Under this condition, it is unable to detect the incidence ofthe next photon. Quenching is to lower the applied voltage to APD tolower than the breakdown voltage, to end the breakdown state, and toenable the detection of the incidence of the next photon. It is calledpassive quenching to simply insert passive elements such as serialresistor to the voltage supply portion and provide such effects, whileit is called active quenching to use an amplifier, etc. to carry outsuch control actively. In the SPCM-AQ, the dead time of the detectorwhich is the time to enable the detector to detect the next photon afterthe previous photon incidence is about 100 ns and the output pulse widthis about 9 ns. Needless to say, a passive quenching photon detector maybe used.

(Description on the Control Method)

Referring now to FIG. 4, description will be made on the operations ofthe differential circuit 3, clock generator 7, and the gate devicecontroller section 8 for controlling the gate device 4 when the idlerphoton 5 is incident on the photon detector 2. In FIG. 4, numeral 18 isa clock pulse outputted from the clock generator, numeral 19 is a graphshowing the incident time of the idler photon 5 on the photon detector2, numeral 20 is a graph showing an aspect of the signal pulsesoutputted from the detector 2, numeral 21 is a graph showing the outputsignal from the differential circuit, and numeral 22 is a graph showingoutput signals, etc. from the gate device controller section.

In the present embodiment, an operation for outputting a light pulsethat contains only one photon during the predetermined time τ from therise-up of the clock pulse shown with numeral 18, and for not outputtingphotons during the period other than that has been achieved.

The idler photon 5 should be so set to provide a sufficiently highprobability of being generated during the timeτ. That is, let N denotethe average number of idler photons generated per second, the idlerphoton 5 shall be set to satisfy N>1/τ. The time τ serves as an index ofthe periodicity of photon output, and in order to increase theperiodicity, τ should be set to be small and N should be set to largeaccordingly.

As the idler photon is incident at each time as seen in numeral 19, asignal pulse series as seen in numeral 20 is outputted from the detector2. For example, if the photon incidence takes place at the time 23, thepulse 25 is outputted from the detector 2 in conformity with this, butfor the photon incidence at the time 24 right after, no pulse isgenerated if it is within the dead time of the detector. The pulse 25generated is converted to a differential signal like numeral 26 by thedifferentiation circuit. For the signal to trigger the gate devicecontroller section, the output pulse 25 from the photon detector may bedirectly used, but using this kind of differential signal 26 as atrigger, it is possible to suppress the fluctuation of the photondetection time due to the fluctuation of the form of signal pulse 25.

In the gate device controller section 8, a control signal 27 for openingthe gate device 4 only for a short time δ is generated in response tothe trigger of the first differential signal 26 after the rise-up of theclock pulse 18. The penetration time at the gate device 4 of the signalphoton 6 corresponding to the idler photon 5 is shown with bars ofdotted lines and solid lines in the graph 22. Because the signal pulseby the idler photon 5 delays by the signal processing time of theelectronic circuit, the signal photon is delayed by the same time with adelay means, but FIG. 4 describes the control method without showingthis. In the gate device 4, the signal photon 28 is able to penetrate,but by shortening the opening time δ of the gate device, it is possibleto suppress the emission of the succeeding signal photon 29 and othersignal photons in the clock. In the next clock, the signal photon 30 isemitted in the similar manner.

This signal photon 6 is focused by the lens 15 and is converged to theoptical fiber 12 while being allowed to pass the filter that selectivelypenetrates the photon of wavelength 2λ. Numeral 14 is a fine alignmentdevice for allowing the signal photon 6 to efficiently incident on theoptical fiber 12.

The length of the optical fiber 12 is set in such a manner as requiredby the time necessary for signal processing as described referring toFIG. 4 so that the photon is transmitted to the gate device 4. The fineadjustment of the time is enabled by adjusting the length of the opticalfiber 12 or by the signal delay unit equipped to the gate devicecontroller section 8, etc.

By the above configuration, there achieved is a single photon sourcethat outputs a light pulse that contains only one photon during apredetermined time τ from the rise-up of the clock pulse and that doesnot output any photon during the time other than the above.

Now, it is extremely useful to output the clock signal 18 shown in FIG.4, signal pulse 25, control signal 27, or differentiation signal 26 tothe outside. The clock signal 18 is able to be used as a signal forcontrolling the total system of the quantum cryptographic communicationsystem. Or it is also possible to provide the clock generator section 7by the clock supplied from the outside or to synchronize.

The photon receiver in the quantum cryptographic communication system isable to receive the signal photon separating from other noise signals byshortening the opening time of the gate device on the receiver side bythe use of the signal pulse 25 or differentiation signal 26.

In the present embodiment, the first photon only from the rise-up of theclock was penetrated, but if a preset counter that is set by a clock 18and is reset when the output pulse reaches the preset number N is used,it is possible to output N pieces of photon in a clock. In such event,it is possible to generate the state in which N photons are contained ina predetermined time.

In the present embodiment, continuous oscillation beam (CW beam) is usedfor the pumping light 10, but pulse beam may be used as a pumping light.In addition, it is also possible to generate the idler photon 5 and thesignal photon 6 more efficiently by installing a mirror for reflectingthe pumping light before and after the nonlinear optical medium 11 andconfiguring a cavity.

Embodiment 2

In Embodiment 1, APD of active quenching control was used as a detector2, but it is also possible to apply voltage for a predetermined time inplace of constantly applying voltage exceeding the breakdown voltage toAPD. Referring now to FIG. 5, the control condition of such case isdescribed. In FIG. 5, numeral 31 is a graph showing changes of voltageapplied to APD with time, numeral 32 a graph showing the signal pulsefrom APD, and numeral 33 a graph showing differentiation signalsoutputted from the differentiation circuit 3.

In Embodiment 1, as described in the section about the active quenching,when the voltage higher than the breakdown voltage is applied to APD,APD has the infinite multiplication to incidence of one photon, and theoutput of APD enters the saturated breakdown state. In the presentembodiment, the voltage applied to APD is controlled in conformity withthe clock 18.

For the period from clock rise-up to about time τ as seen in Numeral 18,the applied voltage is brought to the condition higher than thebreakdown voltage (34). During this period, as soon as a photon toincident, APD enters the breakdown condition, the output is saturated,and the condition persists until the applied voltage becomes lower thanthe breakdown voltage. Consequently, from APD, the output pulse likenumeral 35 is obtained. With the rise-up of the differentiation signal36, the gate device controller section 8 is triggered and the singlephoton is able to be cut out.

When APD of active quenching control is used, it is difficult to shortenthe dead time or pulse length because of the limitation by the circuitused for quenching, and it has been difficult to make the 1-clock timeshorter than the dead time or pulse length of the detector, but thismethod has made it possible to achieve a shorter clock time.

Embodiment 3

In Embodiment 1, the wavelength of signal light generated was 702.2 nm,but needless to say, this wavelength is able to optionally changed byselecting a suitable pumping light source laser and nonlinear opticalmedium. For example, it is naturally possible to generate wavelengths inthe vicinity of 1550 nm, 1310 nm and 800 nm, which are generally adoptedto communication using optical fiber.

The method for generating a photon pair shown in Embodiment 1 (FIG. 3)is a method suited to obtain a photon pair beam with equal wavelengthsand small angular dispersion, but it is possible to obtain a photon pairwith different wavelengths by changing the optical axis direction of theBBO crystal for other object of use. In such event, the two tuningcurves as shown in FIG. 3 come in contact with straight lines thatcorrespond to different wavelengths, respectively. In such event aswell, the photons are taken out at an angle, in which the tuning curvesas shown in FIG. 3 come in contact with the straight lines thatcorrespond to the respective wavelengths. According to this condition,photons that generally expand in a form of cone are converged into onebeam and the photon beam with high distribution density is able to beobtained.

For other embodiments of the invention, there is a device for generatinga 532-nm pumping light 10 using an up-conversion laser of asemiconductor excited Yag laser as a pumping light source 9 in FIG. 2,and generating a 1310-nm photon as a signal photon 6 and a 896-nm photonas an idler photon 5. In such event, by the method described in detailin the Japanese Patent Application No. 353078/1997 “Photon BeamGenerator,” the angle made by the optical axis of the nonlinear opticalmedium is set to such an angle that the tuning curve comes in contact at1310 nm and 896 nm, respectively, in order to improve the photon pairgenerating efficiency. In addition, by setting the wavelength of theidler photon to a near infrared area close to the wavelength of visiblelight, the number of photons is able to be detected with high quantumefficiency of the photon number detector 2.

With this king of configuration, it is able to generate photons in thevicinity of 1310 nm with a small tranmission loss in the optical fiberwithin a specific time τ in such a manner as to prevent two photons fromexisting densely in said time τ. In the present embodiment, it becomespossible to efficiently generate the photon pair by setting the crystalangles as described above, and it becomes also possible to maintain thehigh detection efficiency of the number of idler photons, and as aresult, the power consumption of the device is able to be reduced.

Embodiment 4

FIG. 6 shows another embodiment of the present invention. In thisembodiment, numeral 9 is a pumping light source for pumping theguide-wave channel type nonlinear optical substance 38, numeral 37 anoptical fiber for guiding the pumping light, numeral 39 a wave-guidechannel type nonlinear medium, numeral 41 a wave-guide channel typefilter for discriminating the fluorescent pair and pumping lightgenerated from the wave-guide channel type nonlinear optical medium 39,numeral 40 an emission port of pumping light, and numeral 41 awave-guide channel type filter for dividing the fluorescent pair intotwo branches.

In this embodiment, the parametric fluorescent pair is generated in thewave-guide channel type nonlinear optical medium 39. The fluorescentpair has vertical and horizontal polarized lights, respectively, and inthe wave-guide type filter 41 that operates as a polarizing beamsplitter, that with one of the polarized light pair is transmitted tothe photon number detector 2 and another to the optical fiber 12.

By this kind of configuration, it becomes possible to downsize thedevice, and optical alignment is no longer required.

In this kind of embodiment, as the nonlinear optical medium, awave-guide channel type nonlinear optical medium 39 is used. Asdescribed by “two photon correlation phenomenon by optical wave-guidechannel type nonlinear type element” written by Sanaka et al. in Page341, the second separate volume, No. 2, Volume 53, of the Proceedings ofthe Japanese Society of Physics, in the pseudo phase matching type waveguide channel nonlinear medium, a nonlinearity that can satisfyconditions in which the pumping light used and the photons generated aregenerated in parallel can be obtained by pseudo phase matching.

With this, wavelengths of pumping light and generated photon are able tooptionally chosen.

Of course, in this embodiment, a pulse light source and a CW lightsource can be used as the pumping light source 9. By installing mirrorswhich reflects the pumping light at the front and the rear of theguide-wave channel type nonlinear optical substance 38 and configuringthe cavity, it is possible to produce an idler photon and a signalphoton more effectively.

Embodiment 5

For yet another embodiment of the invention, FIG. 7 shows a case inwhich two shutters are equipped to the gate device 4 of FIG. 2. In FIG.7, numeral 12 denotes an optical fiber for delaying the signal photon inthe reaching time to the gate device, numerals 43, 45 denoteelectro-optic elements, numerals 42, 44, 46 denote polarizing plates,numeral 47 denotes a NOT gate, numeral 48 denotes a delay device, andnumeral 8 denotes a controller. In this event, polarizing plates 44 and46 are set in such a manner that they have the maximum transparency forthe polarization of the light that has passed the polarizing plate 42and do not transmit the light which has polarization crossing at rightangles to said light. In addition, the electro-optic elements 43, 45rotate the polarization 90° when the logic of the control signal givenis 1 and does not rotate the polarization when the logic is 0. It ispossible to configure the shutter by a pair of polarizing plates and anelectro-optic element that rotates the polarization sandwiched betweenthe pair, but in the present embodiment, the polarizing plate 44 is usedin common by the two shutters.

The gate device preferably holds the gate open condition only while thephoton is present, and is closed during other periods. Electro-opticalelements generally have a fast response time by nature. However, with asingle electro-optic element, the gate time was unable to be set to thetime less than that specified by the repetitive response time of theelectro-optic element. The present embodiment has achieved the gateoperation shorter than the repetitive response time of the shutter byinstalling two shutters.

Referring now to FIG. 8, operation of the gate circuit is described. InFIG. 8, time is plotted in abscissa. The graph at the top indicates theprobability in which the desired photon number state reaches the gateoperating section, Graph A the signal at point A of FIG. 7, and Graph Bthe state of signal at point B of FIG. 7 in the similar manner.Denotation A may be considered as the control signal itself from thecontroller. The electro-optical element 45 transmits the photon when thelogic 0 is entered and shields the photon when the logic 1 is enteredin, combinations with polarizing plates 44, 46.

The electro-optic element 43 operates in the same way in combinationswith polarizers 42, 44.

As in the state at time TO of graph A of FIG. 8, the control signal fromthe controller is usually set 1. In this case, the gate device does nottransmit the photon by the electro-optical element 45. In this event, tothe electro-optical element 43, logic 0 is entered by the NOT gate 47,and photon is allowed to be transmitted.

The controller 8 changes the output logic from 1 to 0 so that theelectro-optic element 45 opens at the time T1 right before the time whenthe photon is expected to reach the gate operating section. In thisevent, by the operation of the delay unit 48, the electro-opticalelement 43 is kept to logic 0. In such event, the photon is ready topenetrate the gate device. This state continues for the time set by thedelay unit. After the delay, at T2, the logic to the electro-opticelement flips to 1 and the photon is unable to penetrate the shutterconfigured by the electro-optic element 43. At time T3, the controlsignal again changes from 0 to 1, causing the electro-optic element 45to change to closed state, and at T4, it returns to initial state.

By the above-mentioned configuration, it is possible to open the gatedevice only for an extremely short time, rendering itself capable forselectively emitting the required photons only.

In the present embodiment, the shutter was configured by the use ofelectro-optical elements, but needless to say, it is possible to useother shutters. For example, if a light-light switch is used, stillfaster shutter operation is able to be achieved. When an acousto-opticalelement is used, a shutter faster than the repetition speed is able tobe formed inexpensively. It is also possible to use a mechanicalshutter.

The single photon generating apparatus related to the firstconfiguration of the present invention is able to generate photons of anumber lowering a specific number within a predetermined time after aclock pulse is built up, because it comprises a photon pair source forgenerating a pair of photons comprising a signal photon and an idlerphoton that correlate in the generation time, a photon detector fordetecting the incidence of the idler photon, a clock generator, a gatedevice controller section for generating signals for opening or closinga gate device only by the frequency lowering the specific number oftimes within the predetermined time defined by the clock, and a gatedevice for operating or closing in response to the signal from the gatedevice controller section.

The single photon generating apparatus related to the secondconfiguration of the present invention is able to generate only onephoton within a predetermined time after a clock pulse is built up,because it comprises a photon pair source for generating a pair ofphotons comprising a signal photon and an idler photon that correlate inthe generation time, a photon detector for detecting the incidence ofthe idler photon, a clock generator, a gate device controller sectionfor generating signals for opening or closing a gate device only inresponse to a first signal from the photon detector within thepredetermined time defined by the clock, and a gate device for operatingand closing in response to the signal from the gate device controllersection.

The single photon generating apparatus related to the thirdconfiguration of the present invention is able to efficiently generatephotons of a number lowering a specific number or only one photon withina predetermined time after the clock pulse is built up, because it isequipped with, as a photon pair source, a pumping light source and anonlinear optical medium on which the pumping light is incident, ineither the first or the second configuration.

The single photon generating apparatus related to the fourthconfiguration of the present invention is able to efficiently generate aphoton pair of a specific single wavelength because, as a nonlinearoptical medium on which the pumping light is incident, a nonlinearoptical crystal is equipped, for which the angle made between thepumping light and the optical axis of the nonlinear optical crystal isset to an angle in which tuning curves come in contact with a straightline that corresponds to a specific wavelength a, in the thirdconfiguration.

The single photon generating apparatus related to the fifthconfiguration of the present invention is able to efficiently generate aphoton pair of two specific wavelengths because, as a nonlinear opticalmedium on which the pumping light impinges, a nonlinear optical crystalis equipped, for which the angle made between the pumping light and theoptical axis of the nonlinear optical crystal is set to an angle inwhich tuning curves come in contact with two straight lines thatcorresponds to two specific wavelengths a, b, in the thirdconfiguration.

The single photon generating apparatus related to the sixthconfiguration of the present invention is able to achieve a small-sizesingle photon generating apparatus that does not need any opticalalignment because, as a nonlinear optical medium on which the pumpinglight is incident, a wave-guiding channel type nonlinear optical mediumis equipped, in the third configuration.

The single photon generating apparatus related to the seventhconfiguration of the present invention is able to generate a photon pairin parallel to the pumping light because it is equipped with apseudo-phase matching type nonlinear optical medium as a nonlinearoptical medium on which the pumping light is incident, in any of thethird through sixth configurations.

The single photon generating apparatus related to the eighthconfiguration of the present invention is able to obtain a gate devicethat can open or close in a shorter time than close or open time of theshutter because it is equipped with a plurality of shutters that open orclose in a time difference shorter than the open or close time of theshutter as a gate device for controlling the emission of the signalphoton, in any of the first through seventh configurations.

The single photon generating apparatus related to the ninthconfiguration of the present invention is able to coincide the open orclose time of the gate with the arrival time of the signal photon to thegate device because it is equipped with an optical fiber that allows thesignal photon generated from the photon pair to reach the gate devicefor controlling the emission of the photon, in any of the first througheighth configurations.

INDUSTRIAL APPLICABILITY

The single photon generating apparatus according to the presentinvention is useful as a single photon generating apparatus that is ableto generate photons of a number lowering a specific number within apredetermined time after a clock pulse is built up, because it isequipped with a photon pair source for generating a pair of photonscontaining a signal photon and an idler photon that correlate in thegeneration time, a photon detector for detecting the incidence of theidler photon, a clock generator, a gate device controller for generatingsignals for opening or closing a gate device only by a frequencylowering a specific number of times within a predetermined time definedby the clock, and a gate device for opening or closing in response tothe signal from the gate device controller.

1. A single photon generating apparatus comprising: a photon pair sourceconfigured to generate a photon pair consisting of a signal photon andan idler photon that correlate with a generating time; a photon detectorconfigured to detect an incidence of the idler photon and to generate asignal in response to the detected incidence of the idler photon; aclock generator configured to generate a clock pulse; a gate devicecontroller configured to generate a control signal in response to thesignal received from the photon detector; and a gate device configuredto open or close in response to the control signal received from thegate device controller to control an emission of the signal photon.wherein said gate device opens only for the control signal correspondingto a first occurrence of the detected incidence of the idler photonwithin a predetermined time interval in the clock pulse, saidpredetermined time interval being such a short time as to allow saidgate device to output only a single photon, and to suppress an emissionof a succeeding single photon.
 2. The single photon generating apparatusof claim 1, wherein the photon pair source comprises: a pumping lightsource and a nonlinear optical medium on which a pumping light from thepumping light source is incident.
 3. The single photon generatingapparatus of claim 2, wherein the nonlinear optical medium comprises: anonlinear optical crystal having an optical axis thereof set to apredetermined angle with respect to the pumping light such that tuningcurves come in contact with a straight line having a predeterminedwavelength.
 4. The single photon generating apparatus of claim 2,wherein the nonlinear optical medium comprises: a nonlinear opticalcrystal having an optical axis thereof set to a predetermined angle withrespect to the pumping light such that tuning curves come in contactwith two straight lines having a predetermined wavelength, respectively.5. The single photon generating apparatus of claim 2, wherein thenonlinear optical medium is a wave-guide channel type nonlinear opticalmedium.
 6. The single photon generating apparatus of claim 2, whereinthe nonlinear optical medium includes a pseudo phase matching typenonlinear optical material.
 7. The single photon generating apparatus ofclaim 1, wherein the gate device controller comprises: a plurality ofshutters configured to open and close by a time difference shorter thanan open or close time of the shutters.
 8. The single photon generatingapparatus of claim 1, further comprising an optical fiber configured toallow the signal photon to reach the gate device.
 9. A single photongenerating apparatus comprising: means for generating a photon pairconsisting of a signal photon and an idler photon that correlate with agenerating time; means for detecting an incidence of the idler photon,and for generating a signal in response to the detected incidence of theidler photon; means for generating a clock pulse; means for generating acontrol signal in response to the signal received from the means fordetecting; and means for opening and closing in response to the controlsignal received from the means for generating a control signal tocontrol an emission of the signal photon, wherein said means for openingand closing opens only for the control signal corresponding to a firstoccurrence of the detected incidence of the idler photon within apredetermined time interval in the clock pulse, said predetermined timeinterval being such a short time as to allow said means for opening andclosing to output only a single photon, and to suppress an emission of asucceeding single photon.
 10. The single photon generating apparatus ofclaim 1, further comprising a preset counter configured to be set by theclock pulse and to reset when an output purse reaches a preset number.